1. Field of the Invention
The present invention relates to an artificial socket for use in an artificial hip joint, a fixing screw for securing the artificial socket in an artificial hip joint, and an artificial hip joint using the artificial socket.
2. Description of the Related Art
An artificial hip joint comprises an artificial socket to be embedded and secured in a pelvis, and a stem which is embedded and secured in an upper end of the femur and a tip portion of which stem is provided with an artificial caput capable of being freely rotatably fitted in the artificial socket. Hitherto, a variety of artificial sockets and artificial hip joints, typical examples of which will be described below, have been utilized in the field of orthopedic surgery:
1) a cement bonding-type total artificial hip joint in which a socket and a stem are formed from a plastic material such as polyethylene and a metal material, respectively, and they are bonded with a cementing material such as bone cement, and a cementless bonding-type total artificial hip joint in which no cementing material such as bone cement is used in the bonding of a socket to a stem;
2) a metal-on-metal type total artificial hip joint in which a socket and a caput (bone head) both are formed from a metal material, the socket as a metal cap is supported by a plastic material such as polyethylene, and they are fixed to a human bone with a cementing material such as bone cement; and
3) a ceramic-to-ceramic type total artificial hip joint in which a socket and a caput both are formed from a hard ceramic material, and they are fixed to a human bone with a cementing material such as bone cement.
However, these conventional artificial hip joints have the following problems to be solved.
The cement bonding-type total artificial hip joint has been developed to solve the problems in the artificial caput integrally formed from the caput and stem, both being of a metal material, and thus a socket thereof is also made of an artificial material. As mentioned above, in this type of the artificial hip joint, the socket is formed from polyethylene, the stem is formed from the metal material, and the socket and the stem are secured via the bone cement on the pelvis as a mother base. Since the caput has a small diameter such as about 22 mm and such small diameter limits a movable range of the socket and the caput due to the narrowed latitude of the conformability of the socket with the caput, there is a possibility that a subluxation is caused in the artificial hip joint. Further, there are drawbacks that the deformation of the polyethylene used in the socket can cause loosening and wearing of the hip joint, and that only a low bonding strength can be obtained between the bone cement and the mother base. Furthermore, with regard to use of the artificial hip joint for an extended period of time, there has been made a report that wearing of polyethylene can cause loosening of the joint, because fine powders of the worn polyethylene are distributed around the stem embedded in the femur.
Similar problems are caused in the cementless bonding-type total artificial hip joint using no bonding cement, because the socket of the hip joint is made from polyethylene as in the above-mentioned cement bonding-type artificial hip joint. For this type of the artificial hip joint, deterioration of polyethylene due to its in vivo oxidation can not be avoided. Moreover, there has not been any suggested countermeasure against such wearing of the socket which is caused as a result of the deterioration of polyethylene.
The metal-on-metal type total artificial hip joint has been developed to prevent drawbacks such as reduction of lubricating properties and wearing of polyethylene which are caused due to deformation and oxidative deterioration of polyethylene. For such an artificial hip joint, however, since the socket of the metal material is supported by polyethylene and the joint is bonded through the bone cement to the living body (i.e., the pelvis), it is unable to avoid deterioration of polyethylene due to its in vivo oxidation, and thus the hip joint can not ensure sufficient wear prevention.
The ceramic-to-ceramic type total artificial hip joint is an artificial hip joint in which both of the socket and the caput are formed from hard ceramic material, such as alumina and zirconia. For such a hip joint, since the hard ceramic socket has only an instable strength, it is necessary to largely increase the thickness of the socket in the production of the hip joint. As an essential result, the diameter of the caput used in combination with the socket must be reduced so that it can conform with the configuration of the socket. Further, this hip joint suffers from drawbacks such as a reduced shock-absorbing property than that of the polyethylene-made hip joints and a reduced accuracy in conformation of the socket and the caput than that of the above-mentioned metal-on-metal type artificial hip joint.
In addition to these drawbacks, all the above-described prior art artificial hip joints can not show an adhesion property to the human bone. In particular, for the prior art ceramic-to-ceramic type total artificial hip joint, in order to avoid problems caused due to insufficient strength of the resulting hip joint, it is necessary to remarkably increase a thickness of the socket, in the other words, it is essential to reduce the diameter of the caput in conformity to the increase of the thickness of the socket.